The precise yet dynamic networking among nerve cells is the cellular basis of many if not all brain functions. Alteration or disruption in this network is likely to result in mental deficiencies and/or illnesses. To establish and maintain this neuronal network, neurons adopt a highly specialized and flexible morphology; the formation and modulation of this specialization requires precisely targeted protein/membrane addition to designated plasma membrane domains. A molecule implicated in this targeting process is the Exocyst complex, a macromolecule essential for protein/membrane targeting and critical for neuronal development. Mouse embryos with an Exocyst subunit deletion die upon gastrulation at the onset of neural induction. As a first step to understand the molecular mechanisms of the Exocyst function in neuronal development, we identified the molecular associations of the Exocyst complex and studied its function in neuronal differentiation. We found that the Exocyst complex associates with microtubules and septins, a familty of GTPases whose members have been found to be present in Alzheimer neurofibrillary tangles and act as a substrate for Parkin, a protein implicated in the Parkinson's disease. Septins, in turn, were found to associate with the actin network. Both the Exocyst complex and the septin filament are dynamic molecules which change their subcellular localization upon neuronal differentiation in response to the MAP kinase pathway. In addition, we have also found that the Exocyst complex co-immunoprecipitates with the CDK5 kinase activator p35. We hypothesize that the Exocyst complex coordinates with cytoskeletons, under regulation by signaling molecules such as RalA and the p35/CDK5 kinase system to mediate protein/membrane targeting to designated plasma membrane domains for the generation of neuronal polarity. In this proposal, our objectives are to characterize the Exocyst complex association/coordination with cytoskeletons, to analyze how these interactions contribute to neuronal development and to study the regulation of the Exocyst complex molecular associations and function during neuronal development. These studies aim to further our understanding of the molecular mechanisms and regulations of the Exocyst complex function during neuronal development, and to guide future experimental designs to study the involvement of the Exocyst complex and its associations in neuronal regeneration and degeneration.
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